Easily my eye. Sure it has a creepiness factor, but that's nothing compared to the much more realistic pain factor of the alternative. By the way, i've had three serious injuries to my eyes and twice i had to have one eye glued open and patched so that i wouldn't use that eye while it was recovering.
The achilles tendon. I've injured it before so I know what it feels like. On the other hand, I've never known what it's like to not be able to use one eye, so I'd rather not begin to experience it.
Definitely the eye thing. The conjunctiva (and the cornea) can deal with small incisions like that no problem. Not to mention the fact that there are bugger-all pain receptors there so you shouldn't feel much. Just make sure you don't slice right the way through so your eyeball leaks.
An achilles tendon injury that bad would take absolutely ages to not-quite-heal, even with the best surgery. Plus, you'd almost certainly pass out from the pain before sawing all the way through.
I somehow have this strange feeling in my legs all of a sudden...
I'd go for the eyeballs too, 'cause I know it won't hurt as much (I can put my finger on my eyes without feeling anything, anyone wants to see? contact lenses, gotta love 'em ) and is quite repairable...
However, I think I'd rather go for Murphy's Law once again : if you've got two options which both cause discomfort, a third one will pop up out of nothing